Perl FAQ 4.23
Remember that the substr() function produces an lvalue, that is, it may be assigned to. Therefore, to change the first character to an S, you could do this:
substr($var,0,1) = 'S';
This assumes that $[ is 0; for a library routine where you can't know $[, you should use this instead:
substr($var,$[,1) = 'S';
While it would be slower, you could in this case use a substitute:
$var =~ s/^./S/;
But this won't work if the string is empty or its first character is a
newline, which ``.'' will never match. So you could use this instead:
$var =~ s/^[^\0]?/S/;
To do things like translation of the first part of a string, use substr, as in:
substr($var, $[, 10) =~ tr/a-z/A-Z/;
If you don't know the length of what to translate, something like this works:
/^(\S+)/ && substr($_,$[,length($1)) =~ tr/a-z/A-Z/;
For some things it's convenient to use the /e switch of the substitute
operator:
s/^(\S+)/($tmp = $1) =~ tr#a-z#A-Z#, $tmp/e
although in this case, it runs more slowly than does the previous example.
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